A composite video signal is the sum of a luminance (brightness) signal and a chrominance (color) signal. These signals may are be referred to as luma and chroma signals, respectively. The frequency ranges of the luma and chroma signals are designed to overlap. In video processing, the luma and chroma signal components are added together in order to generate a composite video signal. Integrating the luma and chroma video elements into a composite video stream may facilitate video signal processing since only a single composite video stream is broadcasted. Once a composite signal is received, the luma and chroma signal components must be separated in order for the video signal to be processed and displayed. A comb filter may be utilized for separating the chroma and luma video signal components. For example, a television may utilize a comb filter and may be adapted to receive a composite video input. However, before the television can display the received video signal, the chroma and luma video components have to be separated.
FIG. 1A is a diagram illustrating generation of a conventional composite video signal. Referring to FIG. 1A, a conventional composite video signal 105 may be generated from a luma component 103 and a chroma component 101. The composite video signal 105 may be generated by adding the chroma video signal component 101 and the luma video signal component 103. The chroma signal component 101 may be modulated at 4.43 MHz and it may or may not comprise a constant chroma across the entire line. The luma signal component 103 may increase in amplitude in a stair step fashion or it may not.
A low-pass or band-pass filter may be utilized as a basic luma and chroma separator. Since the color information is in a frequency band centered around 4.43 MHz, the luma signal may be derived by filtering out the color information from the composite video signal with a low pass or notch filter.
FIG. 1B is a diagram illustrating overlapped luma signal 112 and chroma signal 114 in a composite video signal 110. Although low-pass filtering may effectively filter most of the chroma signal 114 out of the luminance signal 112, it may also remove the higher frequency luma signal components. This loss of bandwidth may reduce the horizontal resolution of the luminance signal 112, and fine details in the video picture may be lost. Similarly, the chroma signal 114 may be separated from the composite video signal 110 by using a band-pass filter centered at about 4.43 MHz to obtain only the frequencies between about 3.1 MHz and 5.7 MHz. However, such filtering may not completely filter out the luma present at that frequency and as a result video picture quality may remain limited.
FIG. 2A is a diagram illustrating modulated chroma signals in a composite video frame. The chroma component in a PAL video signal, for example, may be modulated so that a frequency of each successive line of video may be phase-shifted by 90 degrees with respect to the two lines previous. Referring to FIG. 2A, the current frame 201 may comprise two lines previous 203, a current line 205, and two lines next 207. The current line 205 in the current frame 201 may be phase-shifted by 180 degrees from the two lines previous 203 as well as from the two lines next 207 in the current frame 201. In this regard, the two lines previous 203 and the two lines next 207 are in phase.
In conventional video processing of a PAL video signal, there are two ways to separate the luma and chroma video components and these include combing horizontally and combing vertically. During separation of the luma and chroma components, there are two bandwidth directions that may incur losses in the separation process and in the separated signal. Depending on the combing method that is utilized, the separated signal may have reduced vertical bandwidth and/or horizontal bandwidth.
The first way to separate the luma and chroma video components in a PAL video signal is by horizontal combing. Horizontal combing may be accomplished by utilizing a notch filter, for example. Since the chroma signal component in a composite video signal may be modulated at 4.43 MHz, a notch filter set at 4.43 MHz may be utilized. Combing vertically may also be utilized to separate the luma and chroma video components. Combing vertically may be achieved in three different ways—the current line may be combed with two lines previous and two lines next, the current line may be combed with two lines previous, or the current line may be combed with two lines next. The vertical combing is performed spatially, which involves combing only within one field at a time and without any temporal combing.
During combing in the current frame 201, for example, if the current line 205 is added to two lines previous 203, the chroma content may cancel out and two times the luma content may be obtained. On the other hand, if two lines previous 203 is subtracted from the current line 205, the luma content may cancel out and two times the chroma content may be obtained. In this way, luma and chroma content may be separated from the PAL composite video signal for further processing. However, vertical combing in this manner may result in reduced vertical bandwidth.
FIG. 2B is a diagram illustrating combing of a correlated current line and a two lines previous in a current frame. In this case, there is no vertical bandwidth and the two lines previous and the current line are perfectly correlated. The current line may be added with the two lines previous and two times luma may be obtained. Similarly, the two lines previous may be subtracted from the current line so that two times chroma may be obtained.
FIG. 2C is a diagram illustrating combing of a non-correlated current line and a two lines previous in a current frame. In this case, there may be significant vertical bandwidth. The vertical bandwidth may be high enough so that there may be no correlation between the current line and the two lines previous. When the current line and the two lines previous are combed together, there may be significant error in both the luma and chroma. This may produce combing artifacts in the obtained combed video signal. Higher bandwidth in a given direction may cause combing in that direction to result in more incorrectly separated luma and chroma.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the invention as set forth in the remainder of the present application with reference to the drawings.